1. Field of the Invention
The present invention relates to an improvement in a Schmidt trigger or related type circuit which is commonly employed for delaying a signal, for example, in a dynamic MIS memory circuit.
2. Description of the Prior Art
An MIS transistor having a gate electrode formed by polycrystalline silicon is used as a memory cell in a dynamic MIS memory circuit.
Polycrystalline silicon has an appreciably high resistance as compared with Al and other wiring materials. Accordingly, when a drive circuit disposed on one side of a memory cell array is connected with the gate electrode of an MIS transistor forming each memory cell of the memory cell array, for example, in the case of reading them out, information can immediately be read out from the memory cells lying near the drive circuit, but the reading out of information from memory cells situated away from the drive circuit is delayed because the high resistance property of the polycrystalline silicon and the presence of a capacitance cause a time lag in the transmission of a signal to such memory cells from the drive circuit. Therefore, if an output circuit for outputting the read-out information is driven by the same signal as that applied to each memory cell, notwithstanding that the time for reading out the information varies with the position of the memory cell, then accurate information can not be obtained.
To avoid this, there has been proposed such a circuit arrangement as shown in FIG. 1.
In FIG. 1, reference numeral 1 indicates a pre-stage clock generator; 2 designates a memory cell array; 3 identifies an output circuit; 4 denotes a Schmidt trigger circuit; and 5 represents a post-stage clock generator.
The pre-stage clock generator 1 is utilized to convert an external input of the TTL (Transistor-Transistor Logic) level to a MIS level and to generate clock signals .phi..sub.1, .phi..sub.2, . . . (such as shown in FIGS. 1, 2, 3, and 6) for driving other circuits.
The operation of the circuit shown in FIG. 1 can be understood by reference to FIG. 2 which shows the potential variations. The pre-stage clock generator 1, supplied with an external input, applies the clock signal .phi..sub.2 to the gate electrodes of the memory cells of the memory cell array 2, that is, a word line, thus selecting a desired one of the memory cells. Since the word line is made of polycrystalline silicon as mentioned previously, it has a large resistance value and also a parasitic capacitance; accordingly, when a memory cell connected to a far end of the word line is selected, much time is required for applying a sufficient voltage to the memory cell and reading out therefrom information on a bit line at a required level. As can be seen from FIG. 2, an appreciable time lag exists until the potential of the bit line falls sufficiently below a reference level which is dependent on the output circuit 3, after the application of the clock signal .phi..sub.2 to the word line. Consequently, if the output circuit 3 is driven by the clock signal .phi..sub.2, then erroneous information is outputted. To avoid this, in the circuit of FIG. 1 the clock signal .phi..sub.2 is delayed by the Schmidt trigger circuit 4 and then provided to the post-stage clock generator 5 to derive therefrom a clock signal .phi..sub.2 ' which lags the clock signal .phi..sub.2 by a predetermined time and the clock signal .phi..sub.2 ' is applied to the output circuit 3 to activate it, thereby obtaining accurate information.
A specific circuit arrangement of the Schmidt trigger circuit 4 as used in FIG. 1 is shown in FIG. 3.
In FIG. 3, Q.sub.1 to Q.sub.4 indicate first to fourth transistors; R designates a resistor; C identifies a capacitor; N.sub.1 to N.sub.3 denote first to third nodes; A and B represent input terminals; O.sub.T shows an output terminal; Vcc refers to a power source level; Vss indicates the ground level; and .phi..sub.1 and .phi..sub.2 designate clock signals.
This circuit does not pose any problem in its normal operation but has certain disadvantages in case of a variation in the power source level Vcc. This will be described with reference to FIGS. 4 and 5.
Referring first to FIG. 4, a description will be given of the normal operation of the Schmidt trigger of FIG. 3, in which the power source level Vcc does not vary.
Now, let it be assumed that the circuit operates at a power source level Vcc.sub.2 shown in FIG. 4. Upon application of the clock signal .phi..sub.1 to the input terminal B, the transistor Q.sub.1 is turned ON to charge up the node N.sub.1 and its level becomes Vcc.sub.2 -Vth(Q.sub.1), where Vth(Q.sub.1) is the threshold voltage of the transistor Q.sub.1.
When the node N.sub.1 is charged up, the transistors Q.sub.4 is turned ON to charge up the node N.sub.2 and its level becomes Vcc.sub.2 -Vth(Q.sub.1)-Vth(Q.sub.4), where Vth(Q.sub.4) is the threshold voltage of the transistor Q.sub.4.
After the nodes N.sub.1 and N.sub.2 have thus been charged up, the clock signal .phi..sub.1 at the input terminal B assumes a low level and then the clock signal .phi..sub.2 is provided to the input terminal A. As a consequence, the node N.sub.3 is charged up but its rising-up waveform becomes a gentle slope as shown due to the time constant of the resistor R and the capacitor C. This gentle rise due to the time constant achieves a desired time lag.
When the node N.sub.3 is charged up in excess of the threshold voltage Vth(Q.sub.3) of the transistor Q.sub.3, the transistor Q.sub.3 is turned ON, with the result that charges stored at the node N.sub.2 are discharged and its level starts to drop. At this moment, however, since the transistor Q.sub.4 is still in the ON state in which to supply a current to the node N.sub.2, the level of the node N.sub.2 drops in dependence on the gm ratio between the transistors Q.sub.3 and Q.sub.4. Usually, in the case where gm of the transistor Q.sub.3 is 1, gm of the transistor Q.sub.4 is selected about 10, so that the level of the node N.sub.2 falls appreciably gently.
When the level of the node N.sub.2 drops to such an extent that the level difference between the nodes N.sub.2 and N.sub.3 exceeds the threshold voltage Vth(Q.sub.2) of the transistor Q.sub.2, the transistor Q.sub.2 is turned ON and the level of the node N.sub.1 starts to drop.
Thus, in the Schmidt trigger circuit shown in FIG. 3, one of the important functions of the circuit is to slow the discharge at the node N.sub.2 by the time constant (C.times.R) and the current feedback from the transistor Q.sub.4 to the node N.sub.2, thereby achieving a time lag. Because of the current feedback from the transistor Q.sub.4 to the node N.sub.2, the level of the node N.sub.1 does not drop unless the level of the node N.sub.3 becomes appreciably high, that is, unless the level of the node N.sub.3 exceeds the sum of the level of the node N.sub.2 and the threshold voltage Vth(Q.sub.2). In other words, the circuit operates after the level of the word line (see FIG. 2) rises sufficiently high, and accordingly, at that moment the level of the word line rises high above the reference level. It is also an important function of the circuit of FIG. 3 to obtain a sufficient level margin of the bit line by detecting the level of the word line as described above.
Next, a description will be given, with reference to FIG. 5, of an abnormal operation wherein the power source level Vcc fluctuates.
Now, let it be assumed that the circuit operates at a power source level Vcc.sub.1 shown in FIG. 5, where Vcc.sub.1 &gt;Vcc.sub.2.
Upon application of the clock signal .phi..sub.1, the nodes N.sub.1 and N.sub.2 are charged up as in the case described above.
Next, assume that before the application of the clock signal .phi..sub.2, the power source level Vcc.sub.1 falls down to the power source level Vcc.sub.2, Vcc.sub.2 being lower than the charge-up level of the node N.sub.1, as indicated by the broken line D. At this time, however, the clock signal .phi..sub.1 has already fallen and the transistor Q.sub.1 is in the OFF state, so that charges stored at the node N.sub.1 are not discharged on the power source line at voltage Vcc.sub.2 and the level of the node N.sub.1 remains unchanged; in this case, the level at node N.sub.1 stays at the higher level to which the node N.sub.1 was charged up when the power source level was Vcc.sub.1, instead of the power source level Vcc.sub.2. As a result of this, a high bias voltage is applied to the transistor Q.sub.4, increasing a charge-up current to the node N.sub.2.
The gm ratio between the transistors Q.sub.3 and Q.sub.4 is usually about 1 to 10. However, when the current to the node N.sub.2 is large as mentioned above, the apparent gm ratio becomes as large as 1 to 15.about.20, so that even if the clock signal .phi..sub.2 is applied at this time, the level drop of the node N.sub.2 is slowed down, resulting in the discharge of the node N.sub.1 being also delayed.
This delay is indicated by t.sub.1 in FIG. 5. When such a time lag occurs, the delivery of the clock signal .phi..sub.2 ' from the post-stage clock generator 5 is delayed and the generation of information from the output circuit 3 is also delayed, resulting in the access operation being slowed down as a whole.